1. Field of the invention
This invention relates to acoustic pulse generators and more particularly, to a magnetostrictive acoustic pulse generator of the type which can be used at remote or inaccessible locations and which does not require a special high voltage power supply.
2. Description of the Prior Art
Acoustic pulse generators have widespread applications and may be used, for example, in generating sonar pulses for navigation and other submarine applications as well as in well drilling applications such as acoustic formation logging equipment and systems for monitoring the formation ahead of the bit during drilling operations. Although the acoustic pulse generator of the present invention is described in conjunction with drilling apparatuses in a downhole environment, it will be appreciated that the invention is not intended to be limited thereto, as will become apparent.
In downhole logging instrumentation an acoustic pulse generator is used to produce signals of an acoustic frequency adjacent the walls of the borehole at selected intervals to ensonicify and penetrate the walls of the borehole and enable the analysis of the sub-terranian formations through which the borehold passes. Logging systems which are used after a well is completed conventionally employ a wireline cable to supply electrical power from the surface for operation of the necessary circuitry and equipment located downhole. However, in such systems which are used to perform logging or other measuring operations during the process of drilling, electrical power must be obtained from sources located downhole and derived from means such as batteries and mud turbine generators which are incorporated as part of the drill string itself.
One type of acoustic pulse generator employs a piezoelectric crystal to produce mechanical pulses in a fluid medium in response to electical signals. Such devices, however, require very high voltage electrical driving signals, often on the order of 2000 to 3000 volts, in order to produce acoustic pulses of usable strength. Providing such high voltage in a borehole at a location several thousand feet below the surface is both a difficult and expensive undertaking which renders the use of piezoelectric transducers in measuring while drilling systems highly difficult.
Another type of acoustic pulse generator employs a magnetostrictive metal material as a core. Electrical signals are used to produce high levels of electromagnetic flux in the core which, in turn, produce a physical deformation of the core due to magnetostrictive action. This physical deformation is employed to produce an acoustic pulse in the fluid medium within which the core is located. One problem associated with magnetostrictive pulse generators is that in order to produce a sufficient magnetic flux level in the transformer core for magnetostrictive effects to occur, very high magnetic field intensities are required. Again, in a system for measuring while drilling in a borehole environment the provision of downhole power supplies having high current capabilities regardless of whether batteries or mud turbine generators are used, are very expensive to both implement and operate.
The magnetostrictive pulse generator of the present invention overcomes the disadvantages of the prior art by generating he magnetic field intensities necessary for magnetostrictive effects within its own circuitry from power sources of conventional capacity.